Lasers are typically employed in a variety of industrial operations including inspecting, processing, and micro-machining substrates, such as electronic materials. For example, to repair a dynamic random access memory ("DRAM"), a first laser pulse is used to remove an electrically conductive link to a faulty memory cell of a DRAM device, and then a second laser pulse is used to remove a resistive link to a redundant memory cell to replace the faulty memory cell. Because faulty memory cells needing link removals are randomly located, workpiece positioning delay times typically require that such laser repair processes be performed over a wide range of PRFs, rather than at a constant PRF. This production technique is referred to in the industry as on-the-fly ("OTF") link processing and allows for greater efficiency in the rate at which links on a given wafer can be repaired, thereby improving the efficiency of the entire DRAM production process.
However, it is well known that the laser energy per pulse typically decreases with increasing PRF, a characteristic that is particularly true for Q-switched, solid-state lasers. This energy per pulse roll-off limits the upper PRF range for many laser memory repair processes. Moreover, memories and other electronic components are manufactured with various processes each requiring processing by a particular range of pulse energies, often referred to as a "process window." For many memory devices, the "process window" requires that laser pulse energy vary by less than 5% from a selected pulse energy value.
Prior workers have taken various approaches for ensuring operation within a process window or for opening up the process window. For example, U.S. Pat. No. 5,590,141 for METHOD AND APPARATUS FOR GENERATING AND EMPLOYING A HIGH DENSITY OF EXCITED IONS IN A LASANT, which is assigned to the assignee of this application, describes solid-state lasers having lasants exhibiting a reduced pulse energy drop off as a function of PRF and, therefore, a higher usable PRF. Such lasers are, therefore, capable of generating more stable pulse energy levels when operated below their maximum PRF.
It is also known that laser processing applications typically employ positioners to rapidly move target locations on a workpiece through a sequence of programmed processing positions. The movements of the positioner and the laser pulse timing are asynchronous, requiring lasers in such applications to operate in an OTF mode having an inherently wide range of PRFs. The resulting wide range of interpulse periods causes corresponding pulse to pulse energy variations and indefinite pulse firing timing, which leads to inaccurate laser pulse positioning on a workpiece.
Accordingly, U.S. Pat. No. 5,453,594 for RADIATION BEAM POSITION AND EMISSION COORDINATION SYSTEM, which is assigned to the assignee of this application, describes a technique for synchronizing a clock that controls the positioner with a variable clock that controls OTF laser pulse emission. The synchronized clocks allow the laser to emit pulses in synchronism with positioner movements across target locations on the workpiece, thereby improving laser pulse positioning accuracy.
The above-described laser processing applications typically employ infrared ("IR") lasers having 1,047 nanometer ("nm") or 1,064 nm fundamental wavelengths. Applicants have discovered that many laser processing applications are improved by employing ultraviolet ("UV") energy wavelengths, which are typically less than about 500 nm. Such UV wavelengths may be generated by subjecting an IR laser to a harmonic generation process that stimulates the second, third, or fourth harmonics of the IR laser. Unfortunately, the pulse to pulse energy levels of such UV lasers are particularly sensitive to PRF and interpulse period variations.
What is needed, therefore, is an apparatus and a method for generating stable UV laser processing pulse energies at a high PRF in high-accuracy OTF laser processing applications.